This invention relates to a scanning exposure method and a scanning-type exposure apparatus and, more particularly, to a scanning exposure method and a scanning-type exposure apparatus used in a photolithographic process for manufacturing, for example, semiconductor devices, liquid crystal display devices, image pick-up devices (including CCDs), thin-film magnetic heads, and so on.
When manufacturing a semiconductor device or the like using a photolithographic technique, a projection exposure apparatus is used to transfer the pattern on a reticle, which functions as a mask, via a projection optical system onto the respective shot areas of a substrate (i.e., a wafer or a glass plate) coated with a photoresist layer. One of the essential functions of such a projection exposure apparatus is exposure control, i.e., keeping the integral exposure dose on each point on the wafer within an appropriate range.
In a projection exposure apparatus of a collective-exposure type, such as a stepper, a continuous light source (e.g., a super high-pressure mercury-vapor lamp) or a pulsed laser light source (e.g., an excimer laser) is generally used. A collective-exposure type exposure apparatus transfers the circuit pattern of a reticle as a whole onto the shot areas of a wafer, while keeping the wafer stage used for supporting the wafer at rest, unlike a scanning-type exposure apparatus. With either kind of light source, a cut-off control is generally employed as an exposure control device, in which a portion of the exposure light is extracted and guided to a photoelectric detector, called an integrator sensor, during the process of exposing a wafer which is coated with a photosensitive material (photoresist). The integrator sensor detects the amount of light energy to which the wafer is exposed. If a laser light source is used, laser beam emission is continued until the integral value of the exposure energy detected exceeds a predetermined critical level (hereinafter, referred to as the designated amount of exposure dose) required for that photosensitive material. If a continuous light source is used, the shutter is closed when the integral value of the exposure dose detected exceeds the critical level.
With a pulsed laser light source, there is a small variation or error in the energy of the pulsed laser beams. In order to overcome this, the wafer is exposed to a number of pulses greater than the minimum number of exposure pulses to achieve desired exposure-control accuracy. However, if, for example, a wafer coated with a high-sensitivity resist whose designated amount of exposure dose is considerably small is used, the intensity of each pulse of laser beam is high relative to the sensitivity of the resist. If the wafer is exposed to a number of pulses greater than the minimum number, the total amount of exposure dose would exceed the designated amount of exposure dose for that resist. Therefore, in the case in which a material whose designated amount of exposure energy is relatively small is used, an attenuator or the like is positioned in the optical path to attenuate each pulse of the laser beam so as to keep the number of laser pulses emitted to the wafer greater than the minimum number while preventing the exposure dose from exceeding the designated amount.
In recent years, scanning-type projection exposure apparatus, including a step-and-scan exposure apparatus, have been developed, in which a reticle and a wafer are synchronously scanned with respect to the projection optical system so as to transfer a larger circuit pattern imprinted on a reticle onto the shot areas of a wafer with high accuracy. The cut-off control method cannot be used in scanning-type exposure apparatus because the cut-off control method focuses on only one point on the wafer to detect the exposure dose. Therefore, two other control methods have been used for scanning-type exposure apparatus.
The first control method used in the scanning-type exposure apparatus is to simply integrate the quantity of each pulse of the illumination light beam to control the exposure dose. This method is called open exposure control. The second control method is the individual pulse control method, which is disclosed in, for example, Japanese Patent Application Laid-open No. 6-252022. In the individual pulse control method, the exposure dose of a plurality of sub-areas within slit-like exposure areas on the wafer is measured in real time to calculate an integral amount of exposure dose for each pulse of the beam, and the exposure dose of the next pulse of the beam is determined based on the previously calculated integral exposure dose. This method requires a complicated algorithm.
In the first control method, the energy of the pulsed light must be controlled so that the number of laser pulses emitted onto each point of the wafer is an integer in order to achieve a good linearity in the exposure control. The designated exposure amount is expressed as:
(designated amount of exposure dose)=(the number of pulses)xc3x97(average energy of a pulse)xe2x80x83xe2x80x83(1)
The average energy of a pulse is determined immediately before the exposure process from the values detected by the integrator sensor. To this end, an energy modulator is provided in the optical path.
FIGS. 28(A) and 28(B) illustrate examples of conventional energy modulators. The double-grating modulator shown in FIG. 28(A) comprises a pair of grating plates 72 and 74 that are positioned in the optical path of the emitted laser beam LB. Grating 72 is a fixed grating in which transparent portions and light-blocking portions are alternately formed at a predetermined interval. Grating 74 is a movable grating that moves in the direction of the interval. The position of the movable grating 74 relative to the fixed grating 72 is changed to vary the transmittance with respect to the laser beam LB. The modulator shown in FIG. 28(B) comprises a pair of glass plates 76 and 78 that are tilted in the optical path so that the inclination angle "THgr" is variable. An antireflection coating is applied to both surfaces of the glass plates 76 and 78. The transmittance of the glass plates 76 and 78 changes according to the incident angle of the laser beam LB. Taking advantage of this property, the inclination angle "THgr" is controlled to regulate the transmittance with respect to the laser beam LB. As still another means of modulation, the energy of the laser source itself may be modulated.
In addition to equation (1), the scanning-type exposure apparatus must also satisfy equation (2):
V=Ws/Nxc3x97fxe2x80x83xe2x80x83(2)
where V is the scanning speed of the wafer (or the wafer stage) during scanning exposure, Ws is the width of a slit-like exposure area on the wafer, which is referred to as a slit width, N is the number of pulses emitted onto each point of the wafer, and f is the oscillation frequency of the laser.
In the exposure process, the total exposure dose (i.e., the designated amount of exposure dose) required for the wafer is determined first. Then, the average energy of the pulsed light is measured, and the number of pulses emitted onto each point of the wafer is calculated. Finally, the scanning speed V is determined using the slit width Ws and the oscillation frequency f as constants.
In the conventional technique, the oscillation frequency of the laser is fixed at the maximum oscillation frequency f0 based on the maximum scanning speed, which is defined by the capabilities (including mechanical function) of the stage control system of the exposure apparatus.
In scanning exposure, the slit width Ws is a fixed value determined by the optical design of the system, and the oscillation frequency f of the laser is also a fixed value f0, which corresponds to the maximum scanning speed Vmax determined by the capabilities of the stage control system. When the number of pulses emitted to the wafer is the minimum number Nmin (N=Nmin), the scanning speed V is set to Vmax based on equation (2).
As in the collective-type exposure apparatus, so too in the scanning-type exposure apparatus, when a pulsed laser beam is used as the exposure light beam, a number of pulses greater than the minimum number of exposure pulses must be emitted onto each point of the wafer in order to achieve a desired exposure-control accuracy. Therefore, when the designated amount of exposure dose is considerably small, as in the case where a high-sensitivity resist is exposed, each pulse of the laser beam is attenuated by an attenuator (or an energy regulator), which is positioned in the optical path, so that a number of laser pulses greater than the minimum number is emitted to the wafer while preventing the exposure dose from exceeding the designated amount.
An example of the energy regulator for roughly adjusting the exposure dose is shown in FIG. 28(C). The energy adjuster of this example comprises one or more rotatable discs 80, named revolvers, each of which has several ND filters 84 having different transmittances. Transmittance equals (1xe2x88x92attenuation rate). By rotating the respective revolver 80, the transmittance with respect to the incident laser beam LB can be changed to many different values, up to 100%. In the example of FIG. 28(C), the transmittance can be changed to 36 different values (6xc3x976=36). With this energy regulator, the transmittance (i.e., attenuation rate) can be changed only by discrete intervals, normally based on a geometric series.
Because the transmittance can be adjusted only in discrete (not in continuous) fashion, it is often difficult for this type of energy regulator to set the value of the designated attenuation rate to exactly correspond to the designated amount of exposure dose. Therefore, a combination of ND filters that achieves an attenuation rate closest to and less than the target attenuation rate is employed. Consequently, the number N of pulses emitted to each point of the wafer must be further increased in order to take into account the discrete (not continuous) character of the means of adjusting the ND filter transmittance. In other words, the number N of exposure pulses must be increased further beyond the minimum number Nmin to take into account the difference between the target attenuation rate (which could be realized by an ideal continuous-variable energy regulator) and the selected closest attenuation rate. In this case, the scanning speed V cannot be maintained at its maximum, as is clear from equation (2). As a result, the exposure time Texp which equals Ws/V, becomes longer because of the discrete character of the means for adjusting the transmittance of the ND filters, and the throughput of the system drops at certain values of the designated amount of exposure dose. The relation between the designated amount of exposure dose (S0) and the exposure time (Texp) using this type of energy regulator is shown in FIG. 5 by the dashed line.
In general, an illuminance-distribution control mechanism is also provided in a projection exposure apparatus for the purpose of keeping the exposure dose projected onto each shot area of the wafer within an appropriate range.
In a collective exposure type exposure apparatus, the illumination-distribution is controlled using an optical integrator (such as a fly-eye lens), which is positioned in the illumination optical system to form multiple light-source images, thereby superposing luminous fluxes issuing from the multiple light-source images onto the wafer. With a collective-type exposure apparatus, the wafer is at rest when each shot area of the wafer is exposed. Accordingly, the integral exposure dose on each shot area is calculated by continuously receiving a monitoring light flux, which is obtained by splitting a portion of the illumination light during the actual exposure process, by integrating the photoelectrically converted signals of the received monitoring light flux, and by multiplying the integrated signals by a prescribed coefficient, which was obtained experimentally in advance.
Thus, the exposure control mechanism of a collective-type projection exposure apparatus can be easily composed of a photoelectric detector (integrator sensor) for receiving the monitoring light flux, an integrator for integrating the detection signals from the integrator sensor, and a controller for controlling the illuminance of the illumination light (or the exposure time) so that the difference between the integral result and the target value is reduced.
Meanwhile, a deformed light source, in which the aperture of the illumination system aperture stop is off-center from the optical axis for the purpose of improving the focal depth and the resolution for a fine and periodical pattern, has been proposed in, for example, Japanese Patent Application Laid-open No. 4-225358. A zonal illumination method for making the aperture shape of the aperture stop annular has also been proposed. Even if the aperture shape of the aperture stop is varied, the actual illuminance on the wafer surface can be accurately monitored by positioning the light-receiving surface of the integrator sensor on a detection surface that is substantially conjugate with the wafer surface. The exposure time, or the illuminance of the illumination light, is controlled so that the integral value of the detection signals from the integrator sensor is converged to the target value, thereby keeping the integrated exposure dose on each shot area of the wafer within the appropriate range.
A scanning-type projection exposure apparatus, which is often called a slit-scan type or step-and-scan type apparatus, also uses an optical integrator, as in the collective-type exposure apparatus. In the scanning-type of exposure apparatus, if a fly-eye lens is used as the optical integrator, the incident plane of each lens element of the fly-eye lens is conjugate with the pattern surface of the reticle. The illumination area on the reticle has a long rectangular or arc shape, and is often referred to as a slit-like illumination area. In order to improve the illumination efficiency, it is preferable for the lens element to have a long rectangular cross-section similar to the slit-like illumination area.
However, as has been described above, it is difficult to apply the exposure control method used in the collective-type exposure apparatus to the scanning-type exposure apparatus as it is, because, in the scanning-type exposure apparatus, the wafer is scanned relative to the slit-like exposure area formed on the wafer surface (which is conjugate with the slit-like illumination area on the reticle). The exposure dose on each shot area must be controlled so that the integral exposure dose is kept constant over all the points on the wafer while the wafer is passing through the slit-like exposure area. If the integral exposure dose differs among the respective points in the wafer shot area, an error or unevenness is caused in the exposure result of that shot area, which has an effect similar to that of an illumination error or unevenness caused in the collective-type exposure apparatus.
As one of the integral exposure control methods, a shutter is used in a collective-type exposure apparatus, in which the exposure time is controlled by opening or closing the shutter. However, this method cannot be applied to the scanning-type exposure apparatus because the exposure is performed continuously. Therefore, the scanning-type exposure apparatus controls the integral exposure dose by, for example, setting appropriate scanning speeds for the reticle and wafer. It is difficult, however, to change the scanning speeds by a small amount to adjust the integral exposure dose on the wafer in real time during the scanning exposure process.
It is necessary for the scanning-type exposure apparatus to control the illumination light so that the illuminance of the illumination light emitted onto the wafer is stably maintained during the scanning exposure process. One example of such a control method is a constant illuminance control method that constantly monitors the illuminance of the illumination light and feeds the monitoring result back to the power source of the exposure light source to control the electric power supplied from the power source to the light source.
In the scanning-type exposure apparatus, the exposure dose is adjusted according to the sensitivity of the resist. An amount of exposure dose suitable for the sensitivity of the wafer is supplied to the wafer by adjusting the scanning speed, the quantity of illumination light, the width of the slit, or a combination of these factors.
If the output power of the lamp (light source) is p and the attenuation rate of the attenuation plate is q1, the exposure dose e on the wafer is expressed by the following equation using a coefficient k, which is defined according to the shape of the aperture stop in the illumination system:
e=kpq1xe2x80x83xe2x80x83(101)
If the width of the slit-like exposure area in the scanning direction on the wafer surface is D and the scanning speed of the wafer during the scanning exposure operation is Vw, the integral exposure dose xcexa3E on the wafer is expressed as follows in terms of equation (101):
xcexa3E=e(D/Vw)=kpq1(D/Vw)xe2x80x83xe2x80x83(102)
For instance, if the width D of the slit-like exposure area on the wafer is fixed, then the integral exposure dose xcexa3E is proportional to the attenuation parameter r, and is inversely proportional to the scanning speed Vw. This relation is expressed as follows:
xcexa3Exe2x88x9drPmax/Vwxe2x80x83xe2x80x83(103)
From the relation (103), structure for adjusting the integral exposure dose can be grouped into a means for changing the scanning speed Vw and a means for changing the attenuation rate. According to these two groups, the sensitivity of the resist can also be grouped into two regions, that is, a low-sensitivity region (the right half shown in FIG. 29), where the integral exposure dose can be adjusted by solely regulating the scanning speed Vw, and a high-sensitivity region (the left half shown in FIG. 29), where the intensity of each pulse of the beam is adjusted by the attenuator so as to control the total amount of exposure dose. In other words, appropriate integral exposure dose adjusting structure can be selected according to the sensitivity of the resist.
Since the scanning speed can be changed in continuous fashion, the integral exposure dose on the wafer can be finely adjusted according to the sensitivity of the resist in the low-sensitivity region. However, when using an attenuator to change the quantity of exposure light in the high-sensitivity region, it is very difficult to adjust the quantity of exposure light always in continuous fashion because an attenuation plate, which is often used as the attenuator, changes the attenuation rate only in discrete fashion.
For this reason, in practice, it is necessary to adjust the scanning speed for each value of the attenuation rate (or the transmittance) of the attenuation plate so as to correspond to the sensitivity of the resist, as shown in FIG. 30. This method causes the throughput to be reduced.
In order to prevent the throughput from dropping, the discrete values of the attenuation rate (transmittance) must be set finely. This requires an increased number of attenuation plates, which are then assembled in multiple stages. This is inconvenient because the structure of the optical system becomes excessively complicated.
In addition to the problem of the throughput, there is still another problem in that if a high-sensitivity resist is used, the exposure dose must be controlled with higher accuracy than that required by a low-sensitivity resist; otherwise, scanning exposure cannot be satisfactorily performed. This means that an attenuator that is capable of changing the quantity of exposure in continuous fashion is required.
Yet another problem is that an impure gas component in the air may photochemically react with the oxygen, and the reactant may adhere onto the surfaces of lens components of the illumination optical system as a blur, which causes the illuminance to drop. It is known that the blur caused by such a photochemical reaction increases in proportion to the density of the impure gas, the quantity of light, and the illumination time. Some kinds of blur can be removed by rinsing the parts with pure water, but others cannot be removed. In the latter case, the blurred parts must be replaced with new parts. The blur reduces the illuminance of the exposure light onto the wafer, and the lamp (light source) must be frequently replaced. The maintenance tasks and the cost of running the apparatus increase, while the throughput decreases.
In a step-and-repeat type exposure apparatus, the lenses (i.e., glass components) of the illumination optical system are illuminated only when the shutter is open (i.e., only during the exposure process). On the other hand, in a step-and-scan type exposure apparatus, which performs exposure with a constant illuminance, if the illuminance is monitored at a position conjugate with the wafer surface in the optical path under uniform illuminance, the illumination light continuously strikes the lenses not only during the actual exposure period, but even during the stabilizing period for adjusting the illuminance. This increases the blur, as well as the load on the lenses, and the life of the lens is shortened.
In recent years, the illuminance of an illumination optical system has been increased for the purpose of improving the throughput. This situation even worsens the problem of blur mentioned above.
The present invention was conceived in view of these problems in the prior art. It is an object of the invention to provide a scanning exposure method and a scanning-type exposure apparatus that can accurately control the exposure dose. It is another object of the invention to provide a scanning exposure method and a scanning-type exposure apparatus that can precisely transfer the image of a circuit pattern formed on a mask onto a substrate by exposing the substrate to pulsed light, without reducing the throughput. It is still another object of the invention to provide a scanning exposure method and a scanning-type exposure apparatus that can expose a substrate at an appropriate exposure dose regardless of the sensitivity of the resist layer formed on the substrate. It is yet another object of the invention to provide a scanning exposure method and a scanning-type exposure apparatus that can achieve a desired integral amount of exposure dose on a substrate, while keeping the scanning speed of the substrate constant, even if the energy of each pulse of the light source fluctuates during the exposure process. It is still another object of the invention to provide a scanning exposure method and a scanning-type exposure apparatus that can prevent blur from occurring in the optical system. It is yet another object of the invention to provide a scanning exposure method and a scanning-type exposure apparatus that can prolong the life of the light source.
In order to achieve these and other objects, the scanning exposure method according to the invention transfers a pattern formed on a mask onto a substrate by scanning the mask and the substrate synchronously with respect to pulsed light emitted from a pulsed laser light source. This scanning exposure method includes the steps of (i) determining a number of pulses of a light beam that are to be emitted to each point on the substrate during the scanning exposure process, and (ii) controlling the light source according to the determined number of pulses, so that the maximum scanning speeds of the mask and the substrate and/or the maximum oscillation period of the pulsed light are maintained during the scanning exposure process.
The number N of the pulses that are to be emitted on each point on the substrate is determined prior to the scanning exposure process. During the actual scanning exposure process, the pulsed laser light source is controlled so that the scanning speeds of the mask and the substrate and/or the oscillation period of the pulsed light are kept at their maximum based on the determined number N of the pulses.
In the high-sensitivity region, where the designated amount of exposure dose (i.e., the integral amount of exposure dose emitted on the substrate) is small, the designated amount of exposure dose can easily be achieved while keeping the scanning speeds of the mask and the substrate at their maximum, and therefore, a high throughput can be maintained. In this case, the oscillation frequency of the pulsed laser source is controlled based on equation (2) above.
In the low-sensitive region, where the designated amount of exposure dose is large, the synchronous scanning speeds of the mask and the substrate cannot be maintained at their maximum. In this case, the oscillation frequency of the light source (i.e., the oscillation period of the pulsed light) is set at its maximum, and the scanning speeds of the mask and the substrate are adjusted based on equation (2). As is clear from equation (2), if the maximum oscillation frequency is fmax, the scanning speed is (fmax/f0) times as high as the conventional speed, and the exposure time required for exposing a point on the substrate can be reduced to f0/fmax of the conventional exposure time. Because fmax is equal to or greater than f0(fmaxxe2x89xa7f0), the throughput increases even in the low-sensitivity region. If the energy of the pulsed laser source can be changed in continuous fashion, the range of the designated amount of exposure dose that can be used at the maximum scanning speed is broadened, as compared with the conventional apparatus.
In another aspect of the invention, a scanning-type exposure apparatus is provided that includes a pulsed laser source that emits pulsed light along an optical path, a mask stage disposed in the optical path and used for supporting a mask and which is movable in the prescribed scanning direction, and a substrate stage supporting a substrate and being movable at least in the scanning direction. The scanning-type exposure apparatus also has a stage control system communicating with the apparatus components and controlling the scanning speeds of the mask stage and the substrate stage so that the mask stage and the substrate stage are synchronously scanned at a predetermined speed ratio. An alteration system is also provided coupled with the laser source for altering the oscillation frequency of the pulsed laser source. An attenuator is provided in the scanning-type exposure apparatus to attenuate the quantity of pulsed light. The control system controls the alteration system according to the number of pulses that are to be emitted to each point of the substrate when performing scanning exposure using the attenuated pulsed light, so that the scanning speeds of the mask stage and the substrate stage and/or the oscillation frequency of the pulsed laser light are maintained at their maximum.
If the designated amount of exposure dose is small, and if the intensity of the pulsed light emitted by the pulsed laser source is high relative to the designated amount of exposure dose, then the pulsed light is attenuated by the attenuator to keep the number of exposure pulses greater than the minimum number. When the substrate is exposed to the attenuated light during scanning exposure, the number of pulses emitted onto each point on the substrate is varied according to the attenuation rate so as not to drop below the minimum necessary number. Thus, the control system controls the alteration system according to the number of pulses emitted onto each point of the substrate so that the scanning speeds of the mask stage and the substrate stage and/or the oscillation frequency of the pulsed laser source are maintained at their maximum.
In other words, the oscillation frequency f of the pulsed laser source is controlled by the control system according to the number N of exposure pulses to maintain both or either the maximum scanning speeds of the mask stage and the substrate stage and/or the maximum oscillation frequency. For example, in the high-sensitivity region, where the designated amount of exposure dose is small and the necessary oscillation frequency of the laser is not so high, both the mask stage and the substrate stage can be scanned at their maximum speeds during the scanning exposure process, and the throughput is also maintained at its maximum. In this case, the oscillation frequency of the pulsed laser source is controlled based on equation (2) above.
In the low-sensitivity region, where the designated amount of exposure dose for the substrate is large, the attenuation rate of the pulsed light is set to 0% (i.e., the transmittance is set to 100%), and the scanning speeds of the mask stage and the substrate stage are reduced from their maximums. In this case, the oscillation frequency of the laser is set to its maximum, and the scanning speeds of the mask and the substrate are adjusted based on equation (2). As is clear from equation (2), if the maximum oscillation frequency is fmax, the scanning speed is (fmax/f0) times as high as the conventional speed, and the exposure time required for exposing a point on the substrate can be reduced to f0/fmax of the conventional exposure time. Because fmax is equal to or greater than f0(fmaxxe2x89xa7f0), the throughput increases even in the low-sensitivity region. Even if the energy of the pulsed laser source can be changed in continuous fashion, the range of the designated amount of exposure dose that can be used at the maximum scanning speeds is broadened, as compared with the conventional apparatus.
In accordance with still another aspect of the invention, a scanning exposure method for transferring an image of a mask pattern onto a plurality of shot areas of a substrate is provided. In this method, during the process of exposing a shot area on the substrate, integral values of the amount of exposure energy are detected at a plurality of points in that shot area. Based on this detection result, the oscillation frequency of the pulsed light that is to be emitted to the next shot area is determined so that the next shot area is exposed at an appropriate exposure level.
Therefore, even if an unacceptable error occurs in the determination of the amount of integrated exposure dose used to expose a given shot area, the oscillation frequency of the pulsed light is nevertheless controlled correctly based on the integral exposure dose detected during that exposure process, thereby exposing the next shot area at an appropriate level. In other words, the number of pulses is adjusted for the next shot area, without changing the scanning speed, so as to achieve a desired value for the integral exposure dose.
In still another aspect of the invention, the light source and/or the exposure light emitted from that light source are adjusted so that the output level of the light source is set to the minimum required level, and so that the target value of the integral exposure dose is reliably achieved without reducing the scanning speeds of the mask and the substrate from their maximum.
With this method, scanning exposure is performed by moving the mask and the substrate synchronously at their maximum speeds, while setting the output level of the light source at the minimum necessary level. Owing to the low output level, the parts (glass components) of the illumination system can be prevented from deteriorating. As a result, the life of the light source can be prolonged, and at the same time, the throughput can be kept high.
In yet another aspect of the invention, the electric power of the light source is controlled at a constant level in an open loop, as a rough adjustment step, prior to a constant illuminance control step. This rough adjustment step is referred to as a constant electric power control step. On the other hand, in the constant illuminance control step, the illuminance of the light source is precisely controlled at a certain constant level.
In other words, before the illuminance of the light source (e.g., a mercury-vapor lamp) is finely controlled at a constant level, the illuminance is roughly adjusted to near the target value by the constant electric power control step in the open loop. This two-step control can reduce the time required to reach the target value and improve the throughput.
In yet another aspect of the invention, during the process for exposing the substrate to the laser beam, while scanning the substrate, to transfer the mask pattern, the illuminance of the light source is controlled at a constant value. When the exposure process is not being performed, the electric power of the light source is set to a lower level in the open loop. In this arrangement, the power consumption of the light source can be reduced, and therefore, the deterioration of the light source can be prevented. At the same time, the next exposure process can begin smoothly.
In still another aspect of the invention, when the exposure apparatus is not performing exposure (i.e., during the non-exposure period of the operation), the illuminance of the light source is maintained at a constant level, but the exposure light is blocked by a blind so as not to reach the mask or the mask stage. This arrangement precludes the need for readjustment of the exposure dose of the light source, thereby eliminating the time required for adjusting the exposure dose.